This project concerns studies of Rocky Mountain spotted fever and other tick-borne rickettsial diseases in the United States and in certain other countries with emphasis on the ecology, identification, and characterization of rickettsiae and on their relationship(s) to the respective tick vectors. Source material for experimental comparative studies is obtained through collaboration with outside agencies. The project also considers the cellular and subcellular aspects of interaction between tick-borne rickettsiae and their vectors, particularly the mechanism(s) of interference and the factors responsible for changes in the agents' pathogenicity.